This project is a structural examination of the neuronal interconnections in the retinal plexus of the horseshoe crab, Limulus polyphemus. The specific aim has been to explain lateral inhibition in terms of anatomical circuitry. The method entails long, flawless serial sections for electron microscopy combined with tracing and indexing of all arborizations and synapses in a given microglomerulus of the plexus. One specific goal is to correlate spatially graded inhibitory synapses with physiologically derived and currently conflicting curves for lateral inhibition. Once a clear-cut relationship has been established, arborizations in the neuropil can then be assigned to their ommatidia of origin by matrix analysis of their synaptic connections, i.e., by multidimensional scaling. Directly related is a parallel examination of synapses, their fine structure, maturational changes, and possible changes in connectivity. A further objective pertains to the study of octopaminergic and substance P innervation of the eyes and plexus, and the possibility of both substances occurring in the same set of efferent fibers.